In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the unimaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the precursor is positive working. Conversely, if the unimaged regions are removed, the precursor is negative working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque regions. However, direct digital imaging, which obviates the need for imaging through a mask, is becoming increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Thermally imageable elements are disclosed, for example, in Shimazu, U.S. Pat. No. 6,294,311, U.S. Pat. No. 6,352,812, and U.S. Pat. No. 6,593,055; Patel, U.S. Pat. No. 6,352,811; Savariar-Hauck, U.S. Pat. No. 6,358,669, U.S. Pat. No. 6,528,228; West, U.S. Pat. No. 6,090,532; Parsons, U.S. Pat. No. 6,280,899; McCullough, U.S. Pat. Pub. No. 2002/0136961; and WO99/21715; Haley, U.S. Pat. No. 5,372,907; Nguyen, U.S. Pat. No. 5,919,601; Kobayashi, U.S. Pat. No. 5,965,319; Busman, U.S. Pat. No. 5,763,134; and WO 00/17711.
Thermally imageable elements that are to be imaged with infrared radiation, such as with a laser or an array of lasers emitting modulated near infrared or infrared radiation in the range of about 800 nm to about 1200 nm, typically about 830 nm, about 1056 nm, or about 1064 nm, typically comprise an infrared absorber known as a photothermal conversion material. Photothermal conversion materials absorb radiation and convert it to heat.
U.S. patent application Ser. No. 10/384,989, filed Mar. 10, 2003, the disclosure of which is incorporated herein by reference, discloses thermally imageable elements in which the photothermal conversion material is a cyanine dye in which the anion is polysulfonate anion. When these dyes are used as photothermal conversion materials in imageable elements, especially in negative working imageable elements in which the imageable layer comprises a photothermal conversion material, an acid generator, an acid activatable crosslinking agent, and a polymeric binder, the resulting printing plates have improved dot stability. However, these dyes are difficult to prepare and must be prepared by multistep syntheses. Thus, a need exists for a more convenient method for preparing these dyes.